Lecture #9 Date______ Chapter 21~ The Genetic Basis of Development

From fertilized egg to multicellular organism Cell Division: increase in cell number Differentiation: cells becoming specialized in structure and function Morphogenesis; physical processes giving an organism shape

Embryonic development

Differential gene expression Differences between cells come from differences in gene expression (genes turned on or off), not from differing genomes. Evidence: 1- Genomic equivalence: all the cells of an organism have the same genes 2- Totipotency: cells that can retain the zygote’s potential to form all parts of the mature organism (plant cells; cloning) 3- Determination: restriction of developmental potential causing the possible fate of each cell to become more limited as the embryo develops; noted by the appearance of mRNA

Determination--->Differentiation Determination: as the embryo develops the possible fate of each cell becomes more limited Differentiation: specialization of cells dependent on the control of gene expression Induction: the ability of one group of embryonic cells to influence the development of another; cytoplasmic determinants that regulate gene expression Homeotic genes: genes that control the overall body plan of animals by controlling the developmental fate of groups of cells

Genetic cell death Apoptosis programmed cell death (“suicide genes”) 1. Programmed cell death is as needed for proper development as mitosis is. Ex: Reabsorption of the tadpole tail; formation of the fingers and toes of the fetus requires the removal of the tissue between them; sloughing off of the endometrium at the start of menstruation; formation of the proper connections (synapses) between neurons in the brain requires that surplus cells be eliminated.

Apoptosis, Pt. II 2. Programmed cell death is needed to destroy cells that represent a threat to the integrity of the organism. Ex: Cells infected with viruses; waning cells of the immune system; cells with DNA damage; cancer cells

Concept Check Question #1: Why can’t a single embryonic stem cell develop into an embryo? Cytoplasmic determinants: Substances supplied by the mother that influence the course of early development

Cytoplasmic Determinants: Substances supplied by the mother that influence the course of early development Egg cytoplasm contains RNA and proteins encoded by the mothers DNA Heterogeneous unfertilized egg Helps determine developmental fate by regulating expression of the cell’s genes

Determination: as the embryo develops the possible fate of each cell becomes more limited

Concept Check Question #2: If you clone a carrot, will all the progeny plants look identical? No, due to environmental differences Environment also plays a role in the developmental fate

Totipotency in plants: Mature cells in plants can dedifferentiate and give rise to all the specialized cell types of the mature organism Changes in DNA are not irreversible Any plant cell has this ability

Pluripotency of Animal Cells: Adult stem cells are able to give rise to multiple cell types but not all Stem cell: relatively unspecialized cell that can both reproduce itself indefinitely and under appropriate conditions differentiate into specialized cells on one or more types.

Therapeutic Cloning: Using embryonic stem cells to treat disease Harvested from the human blastocyst Reproduce indefinitely into various specialized cells including sperm and egg cells

Concept Check Question #3: The signal molecules released by an embryonic cell can induce changes in a neighboring cell without entering the cell. How? Signal transduction pathway Binding to a surface receptor and triggering a transduction pathway

Induction: signal molecules causing changes in target cells Signals from surrounding embryonic cells can imping on a cell.

Signal molecules: Signal molecules are proteins produced by the growing embryo These molecules send a cell down a specific developmental pathway by causing change in its gene expression that results in observable cellular changes.